Cutler Hammer MV Metal-Clad Arc-Resistant Switch Gear

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Cutler-Hammer VacClad-W Arc Resistant Switchgear

The Result of Innovative Technology

Hugo Sulzer

Switchgear Application Specialist

April 1996

ml Cutler-Hammer



TRADITIONAL METALCLAD SWITCHGEAR CONCEPTS

Developments in Technology for medium voltage metalclad

switchgear in recent years have greatly improved the reliability of the

distribution systems from 2.4KV to 38KV. Vacuum breakers have

fewer moving parts with lower inertia mechanisms resulting from

smaller contact gap requirements. The net result has increased the

reliability factors two fold. Improved insulation materials on breakers

along with bed fluidized epoxy insulation bonded to bus bars have not

only increased the performance reliability but has extended the lifeexpectancy of current designed metalclad switchgear to 40 plus years.

Distribution systems have also moved to higher interrupting ratings

such as 750,1000,1500MVA.

With the trend to higher interrupting ratings, the metalclad switchgear

design is recommended rather than metal enclosed and is the first

choice design of most consultants and utilities. The metalclad design

concentrates on a structure design which reduces the possibility of

arcing faults within the enclosure. For instance, all primary elements

such as breakers, voltage transformers and control power transform-

ers have disconnect means with isolating shutters establishing

isolation from the high voltage source. The enclosures containing

primary elements have been compartmentalized and grounded for

maximum isolation and confinement such as the breaker compart-

ment, main bus compartment and cable compartment. Within these

compartments all live parts where possible are fully insulated reducing

the possibility of an arcing fault to occur. This primary directive to

attempt to eliminate the possibility of an arcing fault has driven the

design development to metalclad switchgear construction for many

years. The design has proven itself to be a reliable switchgear design

in most applications. Structural containment due to arcing faults were

never considered by the traditional standards such as ANSI, IEEE,

NEMA, UL, CSA because of the design criteria established to prevent

arcing faults within the switchgear structure design.

Although arcing faults are rare, injuries from arcing faults in

switchgear have continued. When it does occur, the results can be

very destructive because of the energy levels reached within a

confined compartment. The structural containment proves inadequate

to prevent arcing products and hot gases escaping the faulted

compartment. Burns could result if operating personnel are in close

proximity to the faulted switchgear. Present regulatory organizations

such as NEC, OSHA recognize the hazards of electric arc propagation

and stipulates the use of protective clothing for operating personnel.

REASONS FOR DEVELOPMENT OF AN ARC RESISTANT

SWITCHGEAR

ARCING FAULT PHENOMENA

Arcing faults can occur within a compartment as a result of insulation

failure or human error. The pressure from an electric arc is developed

from two sources: the expansion of the metal in boiling, and the

heating of air by the arc energy. Copper expands by a factor of 67,000

times in vaporizing. This accounts for the expulsion of near-vaporized

droplets of molten metal from the arc; one of the tests showed that

droplets could be propelled up to 10 feet. The pressure also generates

plasma outward from the arc for distances proportional to the arc

energy. One cubic inch of copper vaporizes into 1.44 cubic yards of

vapor. The air in the arc stream expands in warming up from its

ambient temperature to that of the arc temperature (approx. 35,000

degrees F). All this happens within the first half cycle of the fault and

results in a sudden, large rise in pressure inside of the compartment.

The structure of the switchgear offers some containment but may not

be enough to prevent personal injury.

HISTORY OF ARC RESISTANTSWITCHGEAR IN EUROPE

In Europe, concern for internal arcing within the enclosure of

switchgears had existed for a number of years and in April 1969, work

in Germany led to the publication of “Pehla Recommendation no. 2”

which described the method for testing switchgear under conditions of

internal arcing and gave the criteria for accepting an arc resistant

construction. Some years later an IEC working group was formed to

study a German proposal for amending the switchgear specification

publication 298 to include a section on non-mandatory internal arc test

and in December 1978, Amendment no. 2 to IEC publication 298 was

adopted.[l] This Amendment and subsequent updates are considered

as the basis for arc resistant testing of metalclad switchgear in

Europe. It gives the locations where internal faults are more likely to

occur, types of accessibility for a switchgear, test arrangements, test

current and voltage to be applied, test procedure and the criteria for

assessing the test results. This IEC standard was also used as the

base standard in the development of the EEMAC Standard G14-1

1987 in Canada.

CANADIAN DEVELOPMENT OF ARC RESISTANT SWITCHGEAR

To provide some background in Canada as to how the EEMAC

Standard was developed, the following events took place before the

Standard was written. A failure took place in a substation in the City of

Toronto. The racking mechanism for the incoming circuit breaker was

designed such that the circuit breaker could pivot slightly out of

alignment and still satisfy interlocking requirements. This resulted in a

poor connection in one of the outer phases at the circuit breaker upper

primary disconnect.

The disconnect overheated, resulting eventually in thermal breakdown

of the circuit breaker bushing, and a flashover to the wall of the circuit

breaker cubicle. The resulting explosion in the circuit breaker

compartment caused the front door to bow, fractured its upper and

lower fastenings, and swung the door open, ejecting hot arc products

into the station building. Nobody was close to the equipment at the

time and there were no injuries.

In another event in Ontario’s Niagara Region, due to changes in

metering requirements in the Region, some current transformers were

removed from a circuit breaker cell and replaced by epoxy insulated

copper bars. One of the bolted connections was not properly torqued

and in the course of about a week, the heat generated at the poor

connection caused the insulation to deteriorate to the point of failure.

Page 2



e flashover resulted in a cable compartment cover being blown off,

aring eleven fastening bolts. Again, nobody was close to the

ipment and there were no injuries,

e third event had two explosive failures in adjacent cable compart-

nts in a metalclad switchgear in a Hamilton transformer station.

e primary cause of both failures was attributed to snow blowing into

cubicles via a ventilation louvre under the building eaves, resulting

he eventual failure of two cable potheads.

e first fault was interrupted by the feeder breaker but resulted in the

ors of the cable compartment being blown open. Five workmen

uding supervisors were dispatched from Ontario Hydro and

milton Hydro to examine the failed equipment and to start the repair

cedures. As they gathered around the failed cell, the second fault

he adjacent compartment took place. Because of a faulty feeder

uit breaker, the second fault persisted for a longer period of time,

h the result that the doors were completely torn off and the hot arc

ducts spilled into the aisleway. Four of the workmen were seriously

lowing this last failure, it was decided future metalclad bought by

tario Hydro must be able to withstand the explosive forcesnerated by faults inside metalclad cells i.e., the design must be arc

istant. Prior to these failures, Ontario Hydro had been made aware

European developments in this area, particularly in Germany. At the

e of Ontario Hydro’s decision, the International Electrotechnical

mmisssions (IEC) were in the process of establishing criteria for

cessful type testing of an arc resistant design. These were later

blished in Amendment No. 2 to IEC Standard 298 as previously

ted.

tario Hydro directed the members of the Electronic & Electrical

nufacturers Association (EEMAC) to form a working group to write

imilar Canadian specification to deal with the proper procedure for

ing a switchgear design which would prevent explosive forces from

caping due to the failure of the structure containment during the

ere overpressure phase of a fault.

e specification was completed in 1987 heavily influenced by Ontario

dro. The basis for the EEMAC G14-1 test procedure has similar

eria as established by IEC but strengthened in areas where Ontario

dro felt the procedure for testing needed improvement from the

ropean design criteria at the time it was written. To date, IEC has

de modifications to their specifications to improve the safety of

ntrol gear manufactured in Europe.

SIGN LEVELS FOR ARC RESISTANT SWITCHGEAR

e EEMAC G14-1 although a test procedure, does define three

inct levels of arc resistant design corresponding to the test

nditions stipulated within the test procedure.

Accessibility Type A: Switchgear with arc resistant

construction at the front only.

Accessibility Type B:

‘\

Accessibility Type C:

(Utility Requirement)

Note to Type C:

Switchgear with arc resistant

construction at front, back and sides.

Switchgear with arc resistant ’construction at front, back andsides

and between compartments within the

same cell or adjacent cells.

The only exception is that a fault in a

bus bar compartment of a feeder cell isallowed to break into the bus bar

compartment of an adjacent feeder cell.

(This recognizes the fact that most bus

compartments have very little volume

for gas expansion and that the pressure

relief in breaking into the adjacent main

bus compartment is acceptable,

according to EEMAC G14-1 1987.)

EVALUATION CRITERIA OF A SUCCESSFUL TEST

The test procedure outlines the following stipulations which must be

met for evaluating an acceptable arc resistant design.

Criteria No. 1

That properly secured doors, covers, etc., do not open.

Criteria No. 2

That parts which may cause a hazard do not fly off. This includes large

parts or those with sharp edges, for example, inspection windows,

doors, pressure relief flaps, cover plates, etc. made of metal or plastic.

Criteria No. 3

Accessibility Type B:

Accessibility Type C:

That arcing does not cause holes to

develop in the accessible front of the

switchgear.

That arcing does not cause holes in the

freely accessible front, sides and rear

of the enclosure.

That arcing does not cause holes in the

freely accessible front sides and rear of

the enclosure or in the walls separating

the cells in an assembly (except for

main bus bar barriers) or between

compartments of a cell. ’Criteria No. 4

That the cotton indicators fitted as per test specification do not ignite.

Indicators ignited as a result of the burning of paint, labels, etc. are

excluded from this assessment.

Page 3



EEMAC standard requires the cotton indicators to be 150g per square

meter density and must be 10.2 cm from the test surface.

IEC standard requires 150g per square meter density and must be

30 cm from test surface.

Criteria No. 5

That all the grounding connections remain effective.[3]

With the above five criteria having been tested and assessed by an

independent high voltage test station, a manufacturer is deemed to

have an arc resistant design.

At this point in time, it must be emphasized that failure within the

switchgear enclosure due either to a defect, an exceptional service

condition as an example, corrosive atmosphere or mal-operation may

initiate an internal arc.

There is little probability of such an event occurring in equipment

meeting the requirements of ANSI, IEEE, EEMAC but it cannot be

completely disregarded. Such an event may lead to the risk of injury,

if persons are present in the vicinity of the equipment.

It is desirable that system designers and purchasers provide the highest

appropriate degree of protection to persons. The principal objective is to

avoid internal arcs or to limit their duration and consequences.

Experience has shown that faults are more likely to occur in some

locations inside an enclosure than in others. Special attention should

be paid in such areas. For guidance, a list of such locations and

causes is given in Columns 1 and 2 of Table 1 of EEMAC G14-1.

Measures to decrease the probability of internal faults or to reduce therisk are recommended but not limited to examples in column 3.

If the measures described above are considered to be insufficient, then,

to cover the case of an arc occurring entirely in air within the switchgear

enclosure, a test in accordance with EEMAC G14-1, 1987 may be

agreed between the manufacturer and user. The tests required make

allowance for internal overpressure acting on covers, doors, inspection

windows, etc. and also takes into consideration the thermal effects of the

arc or its roots on the enclosure and of ejected hot gases directly from

the switchgear and damage to partitions which would endanger

operating personnel doing maintenance inside adjacent compartments.

It does not cover all effects which may constitute a risk, such as toxic

gas nor the location of the equipment within a building.

Table 1 Locations, causes and examples of measures decreasing the probability of internal faults or reducing the risk [3]

Locations where internal faultsare more likely to occur Possible causes of internal faults Examples of Measures

1 2 3

Cable Termination Inadequate design Selection of adequate

Compartments dimensions

Faulty installation Avoidance of crossed cable connections.Checking of workmanship on site.

Disconnectors SwitchesGrounding Switches

Failure of solid or liquid insulation (defectiveor missing)

Mal-operation

Check of workmanship and/or dielectric teston site. Regular checking of liquid levels.

Interlocks. Delays re-opening. Independentmanual operation. Making capacity forswitches and grounding switches.Instructions to personnel

Bolted Connectionsand Contacts

Instrument Transformers

Corrosion

Faulty assembly

Ferroresonance

Use of corrosion inhibiting coatings and/orgreases. Encapsulation where possible.

Checking of workmanship by suitable means.

Avoidance of these electrical influencesby suitable design of the circuit.

Circuit Breakers Insufficient maintenance Regular programmed maintenance.Instructions to personnel.

All Locations Error by personnel Limitation of access by compartmentation.Insulation embedded live parts.Instructions to personnel.

Aging under electric stresses

Pollution, moisture,dust vermin etc.

Partial discharge routine tests.

Measures to ensure that the ingressspecified service conditions are achieved.

Page 4



TLER-HAMMER VacClad-W ARC RESISTANT SWITCHGEAR

VELOPMENT

new Cutler-Hammer using the Westinghouse VacClad-W medium

age arc resistant switchgear design provides more advanced

hnology and more flexibility by incorporating into the basic steel

cture all requirements of containment during an arcing fault,

ing the many studies and tests conducted by Cutler-Hammer, its found that the internal arcing phenomenon consists of two stages

ally, the dynamic phase and a thermal phase. See Figure 1.

E DYNAMIC PHASE

The overpressure and the magnitude of arcing current and the volume

of the compartment are all interrelated. There are many differential

equations which have been developed but the geometry of each

switchgear compartment are subtly different making actual testing the

only ultimate way to prove an arc resistant design. The issue here is

to design a pressure relief vent into the switchgear compartment

design to allow the dynamic phase to dissipate without losing the

integrity of the fastening devices.[4]

THE THERMAL PHASE

While the arc is burning and expanding, part of the compartment bus bars

will vaporize, insulation will melt and disintegrate and burning of paint

results in smoke and fumes. The longer the fault is allowed to persist

beyond 30 cycles, the arc could burn through steel. This is why relay

coordination settings are very important to clear the fault before burn

through during the thermal phase takes place. Understanding the

dynamics of an arc fault and energy levels attained allowed Cutler-

Hammer to achieve the design goals required for arc resistant switchgear.

he start of arc initiation within 10 milliseconds, the absolute

ssure inside a switchgear enclosure could reach a pressure level

232 Ibs/square foot in some instances but this value is a function

he fault current magnitude. With such a rate of rise of pressure,

tainment cannot be accomplished within the compartment.

ure 1 Pressure in a cell/compartment during arc.

** *

I I I i

16 36 48 64 80 96 112 128 144 160I I I I I I I I I I

Time (ms)

Illustration of a three-phase internal arc fault

A) Short circuit current B) Arc Voltage C) Dynamic pressure inside the cell

* Compression

* Expansion

* Emission

* Thermal

Page 5



ESTING PROGRAM above the switchgear must be considered since the arc energy is now

being focused through the tops of the switchgear. Compartment to

utler-Hammer designed pressure relief vents in sufficient cross compartment as defined by EEMAC G14-1 had to be tested so that

ection areas on the roof of each metalclad compartment to effectively ( the design levels A, B and C were proven to be safe. The pressure

elieve the stresses within each compartment. The critical design of relief vents on top of the switchgear had to be of suitable thickness to

ourse are the hinges and latches which must hold together while ensure a walkable roof. During the installation of the switchgear,

ressure relief is accomplished. The overlaps of steel flanges had to construction crews quite often walk on top of enclosures during

eal off hot gases so that the cotton indicators outside the switchgear offloading and placing into final position on the switchgear floor.

ould not ignite. Cotton indicators representing bare skin in close

roximity to switchgear compartments in a typical test are shown in The Cutler-Hammer design not only has a walkable roof but theigure 3. The squares represent cotton indicators strategically placed unique raised flange provides automatic dripproof construction with

vulnerable areas of the switchgear design. Establishing a clear area every arc resistant design,

igure 3

FRONT VIEW REAR VIEW

age 6



test shown in Figure 4 shows the set up of cotton indicator

ions. The main bus compartment will be shorted with a copper

having 0.5mm diameter sufficient to initiate the arc. These test

ences were performed with type c arc resistant construction.

re 4

3

F R O N T 0

“IEWA_A

RWIRE 0

r’A B.5 mm

NOTES:?-TEST TO BE CARRIED OUT WITH ALL DOORS ON CELL NO. 3 OPEN

ItI

“a

LEFT SIDE VIEW REAR VIEW

Page 7



The design in Figure 5 is an internal bus duct rated 3000A. Bus duct

connections will be necessary in some form and testing to validate the

arc resistant design is necessary. Outside the compartment bus duct

location of cotton indicators to detect any leakage of hot gases is

critical even to extend beyond the roof line where the pressure vents

are located.

Figure 5

PLAN VIEWroof

NOTES:1. TEST TO BE CARRIEDOUT WITH DRESS PANELSREMOVED.2. CATU GROUNDING STUDSNO LONGER REQUIRED ININTERCELL BUS DUCT.

FRONT VIEW SIDE VIEW REAR VIEW

Page 8



LT LEVELS AVAILABLE

procedure for testing as written in EEMAC G14-1 states that the

levels tested must be agreed between the manufacturer and end

. The actual fault level is not stipulated. To provide improved

ator safety, the Cutler-Hammer design was tested to match the

rupting rating of the breaker. This ensures a coordinated design

e 2

between arc resistant ratings and breaker ratings for the life of the

switchgear design. It is common practice to add more feeder load on

distribution switchgear including motor feeders. Motor contribution will

increase the fault capacity of the distribution system. The allowed

increase of course must not exceed the interrupting capacity of the

breaker and for this reason, arc resistant tests were performed

matching the breaker ratings. See Table 2.

Nominal Rated Rated Rated Short Maximum Arc Resistant

cuit Breaker Nominal 3-Phase Rated Voltage Continuous Circuit at Rated Symmetrical Short Circuitpe and Impulse Voltage MVA Maximum Range Current at Maximum Interrupting Current Levelvel Class Class Voltage Factor K 60 Hz Voltage Capacity at 60 Hz

VCP-W250 4.16 250 4.76 1.24 1200 29 36 36kV B.I.L. 2000

3000

VCP-w350 4.16 350 4.76 1.19 1200 41 49 49kV B.I.L. 2000

3000

VCP-w500 7.2 500 8.25 1.25 1200 33 41 41

kV B.I.L. 20003000

0 VCP-w500 13.8 500 15 1.3 1200 18 23 23kV B.I.L. 2000

3000

0 VCP-w750 13.8 750 15 1.3 1200 28 36 36kV B.I.L. 2000

3000

0 VCP-WlOOO 13.8 1000 15 1.3 1200 37 48 48kV B.I.L. 2000

3000

0 VCP-W25 27 1170 28.5 1 630 25 25 255kV B.I.L. 1200

2000

Page 9



CONSTRUCTION DETAIL HIGHLIGHTS AND DIMENSIONS

All operations performed on breakers, potential transformers and

control power transformers, engagement/isolation/test are with the

compartment doors closed for operator safety. The front door is

nterlocked with the shutter assembly to reduce the chance of

accidental opening of the front door during even partial levering-in of

he breaker/potential transformer/control power transformer drawout

element. Viewing windows are provided so that the operator is able to

see at all times what position the drawout element has reached such

as the connected position or test/isolated position. In any breakerposition, the status indicators on the breaker can be seen through the

viewing windows which include breaker open and closed indicator

lag, stored energy mechanism charged or discharged flag. All

charging of stored energy mechanisms are done with the front door

closed in full view through the Lexan viewing windows, For typical

dimensions. see Table 3.

A cautionary note must be made that all doors and panels must be

properly closed and fastened for the arc resistant feature of the

switchgear to be operative.

The rear cable compartment has two designs available; an all bolted

back panel or a rear door with an 8 point latching handle mechanism

which requires no bolting.

The walkable roof combined with the inherent dripproof construction

provides the highest degree of equipment protection during installation

unctions such as levering-in, manual trip, manual close, manual and commissioning

Table 3 Typical Dimensions Indoor and Outdoor VacClad-W Arc Resistant Switchgear

I I I

Circuit BreakerType and ImpulseLevel

50 VCP-W25060kV B.I.L.

50 VCP-w35060kV B.I.L.

NominalVoltageClass

4.16

Nominal Rated3-Phase ContinuousMVA Current atClass 60 Hz

75 VCP-w500 7.2 500 120095kV B.I.L. 20001 I I 3ooo

\ L

150 VCP-w500 13.8 500 1200

150VCP-w750 13.8 750 120095kV B.I.L. 2000

3000

92.38”96.38”96.38”

150 VCP-WI000 13.8 1000 120095kV B.I.L. 2000

3000

92.38”96.38”96.38”

270 VCP-W25 27 1170 630 92.38”125kV B.I.L. 1200 96.38”

2000 96.38”

I I I

T

Height

92.38”96.38”96.38”

92.38”96.38”96.38”

92.38”96.38”96.38”

92.38”

96.38”96.38”

J

Indoor

Bottom

- - l - -

EntryWidth Depth

36” 97”97”

109”

36” 97”97”

109”

36” 97”97”

109”

36” 97”

97”109”

36” 97”97”

109”

36” 97”97”

109”

I

42” 109”

r Outdoor 1TopEntryDepth

Add 3”plus TopCableSpace


Add 3”

plus TopCableSpace

Add 3”

plus TopCableSpace



Add 3”plus Top

CableSpace

Height Width

128” Add 4”per Cellto IndoorDimension



128” Add 4”

per Cellto IndoorDimension



128” Add 4”per Cell

to IndoorDimension

Bottom TopEntry EntryDepth Depth

Add 10” Add 70”to IndoorDimension


Add lo” Add 70”to IndoorDimension

Add 10” Add 70”

to IndoorDimension



Add 10” Add 70”to Indoor

Dimension

Page 10



e 4 Section Views

Section view 5kVand 15kV; 1200A VacClad-WArc Resistant switchgear cubicle

Section view 27kV; 1200A VacClad-WArc

Resistant switchgear cubicle


Section view 27kV, 12OOA VacClad-WArc

Resistant switchgear cubic/e


Standard outdoor arrangements available in non walk-in, walk-in and common aisle (walk-in shown)

CLUSION

reliability of Westinghouse switchgear and circuit breakers, now

r-Hammer, has been proven by over 25 years of vacuum interrupter

n and manufacturing experience. The ongoing research and

opment program has resulted in many significant advances in

um interrupter technology. These advances have been incorporated

he Cutler-Hammer VacCiad-W Arc-Resistant Switchgear and VCP-W

it Breakers to provide enhanced dependability, reliability, and safety.

nsive design and development taking into account all critical mechani-

riteria of impact under overpressure makes the Cutler-Hammer

Arc Resistant switchgear the best design available in the

etplace. Cutler-Hammer provides VacClad-W Arc Resistant switchgearh meets most utility design requirements. Multiple bolting, reinforced

heavier gauge material does not provide a better design or longer life

ctancy; it only adds more weight to the switchgear assembly and more

plexity to servicing. The Cutler-Hammer VacClad-W Arc Resistant

n is not a retrofitted general purpose switchgear cubicle. Compartment

ed steel design achieves panel against panel interfacing, providing a

d joint under fault conditions which prevents smoke and gas escaping

her compartments, instead of conventional flat bolted panels which are

ecessarily smoke or gas tight.

The switchgear cubicle can be easily removed even after a major fault

occurrence without disturbing the adjacent cubicle, even in cases

which have inaccessible locations. The enclosure can be dismantled

inwardly with minimal unbolting.

Greater life expectancy is achievable with the Cutler-Hammer design

with minimum down time if replacement or repair is ever necessary.

REFERENCES

[l] L. Lam “Development and Testing of Arc-Proof Metal-Clad

Switchgear for 25kV and 34.5kV Application” presented at CEA

Fall meeting Sept 1982 Edmonton, Alberta.

[2] J.P. Meehan “Arc Proofing of Metal-Clad Switchgear-A Utility’s

Viewpoint” Ontario Hydro 1980-81.

[3] “Procedure for Testing the Resistance of Metal-Clad Switchgear Under

Conditions of Arcing due to an Internal Fault” EEMAC G14-1, 1987.

[4] Michel G. Drouet, Francois Nadeau “Pressure Waves due to

Arcing Faults in a Substation” IEEE transactions on power

apparatus and systems, Vol. PAS-98, No 5, Sept/Oct 1979.

Page 11



ABOUT THE AUTHOR

Hugo Sulzer received the Engineering Technologist degree in

Electrical/Electronics from the Hamilton Institute of Technology in

Hamilton, Ontario, Canada. He is a Switchgear Application Specialist

working in the Cutler-Hammer Sales Department in Canada. Hugo is

also a national resource for applications with vacuum devices in

Canada and is an associate member of IEEE. Hugo has been involved

with the development of Arc Resistant Switchgear for Cutler-Hammer,

Canada from its early design to the present.

Cutler-Hammer Westinghouse &

Cutler-Hammer ProductsFive Parkway CenterPittsburgh, PA 15220

(412) 937-6100

SA-233

Printed in U.S.A.

Documents

Cutler Hammer MV Metal-Clad Arc-Resistant Switch Gear